10 minutes with Olivia Williams

Olivia Williams. In a Forest, Dark and Deep is at the Vaudeville Theatre until 4th June (0844 412 4663)

Olivia Williams is sweeping bottles of foundation into a make-up bag by her dressing-room mirror. 'I feel like I should tidy up,' she apologises.

We are in the bowels of the Strand's Vaudeville Theatre, which first opened in 1870 to stage racy shows. Olivia, who is a little hoarse after the first three-week run of Neil LaBute's psychological drama In a Forest, Dark and Deep, is still in costume: a denim shirt, black jeans and white sneakers. 'It was very intense today. We have to fight on stage and Matthew [Fox, of Lost fame] is very strong. I've had to tell him to be gentle with me. The last time I came close to breaking a rib was during childbirth and I really don't want that to happen again.'

Olivia describes her dressing room as 'palatial', although it strikes me as rudimentary. 'I have a shower. The height of luxury in the West End.' She points out the bed where she takes regular power naps and then leads me down some stairs, past ominous signs saying things such as 'The wardrobe is watching you'. Backstage we have to whisper as the understudies are rehearsing.

There is no messing about on stage. 'We're not pranksters. You can't be when it's just the two of you. If you're in an RSC production and there are 32 people on stage and you're one of the spear carriers, then you can f*** about.' I'm completely disoriented but Olivia points out that we're facing the audience. 'We're at the back of the A-frame house.' She shows me the well-stocked prop table. 'Here are the joints we smoke and the beers that Matt drinks,' and some of the theatre's old-fashioned touches: 'Look at this fire cord with the old rusty hammer, you cut the cord and the curtain comes down. It's so romantic.'

Once we're in the auditorium, Olivia leaps on to the edge of the stage in front of the safety curtain. 'The stage goes up on a slope - you've got to direct your performance to the fire alarm right at the back because there's some poor bastard sitting up there.' I ask whether she dissects her performances once she's offstage. 'We don't really get a chance to break down what's happened. It's a bit like going down the Cresta Run - you're not really conscious while you're doing it and then it's over.'

Before I leave, Olivia insists that I see the show. Luckily, I'm not the poor bastard up in the gods, but I can understand why she needs that bed. In a Forest, Dark and Deep is at the Vaudeville Theatre until 4 June (0844 412 4663)